ksz884x: don't copy too much in netdev_set_mac_address()

MAX_ADDR_LEN is 32.  ETH_ALEN is 6.  mac->sa_data is a 14 byte array, so
the memcpy() is doing a read past the end of the array.  I asked about
this on netdev and Ben Hutchings told me it's supposed to be copying
ETH_ALEN bytes (thanks Ben).

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>